1. Field of the Invention
The present invention relates generally to the field of motion sensors and, more particularly, to a new sensor having a combination of low frequency sensitivity and immunity to damage from mechanical shock and mechanical fatigue.
2. Description of the Prior Art
Motion sensors are used in a wide variety of applications and as a result many different types of motion sensors are available. They may be used to trigger the deployment of airbags in automobiles and to measure the vibration of machinery. In these examples the motion is of a relatively high amplitude and high frequency. The sensors available for such applications are rugged and nearly immune to failure from mechanical shock or mechanical fatigue. Motion sensors are also used in oil and gas exploration to measure the movement of the Earth itself. In this case the motion is of a much lower amplitude and lower frequency than in the previously mentioned examples. Such sensors have high rates of complete failure and degraded performance from mechanical shock and mechanical fatigue experienced in transportation and operation.
Almost all motion sensors use a spring and mass to detect motion. Under acceleration the spring will deflect and the mass will move in proportion to the frequency and amplitude of the acceleration. The movement of the mass is converted to an electrical signal that is representative of the frequency and amplitude of the acceleration. In the case of airbag sensors and machinery vibration sensors the spring is a mechanically sturdy device that is not easily subject to damage. However, the requirements imposed on motion sensors for use in oil and gas prospecting preclude the use of a sturdy spring. These motion sensors, commonly called geophones, have a pair of thin delicate easily damaged springs. This type of spring cannot be avoided if the geophone is to have the required low frequency response and sensitivity to low amplitude motion. Much effort has been expended in seeking alternatives to the conventional geophone but to date none of the results have met with significant acceptance by the industry.
The present invention substitutes liquid movement for the movement of a spring suspended mass. The present invention is a geophone with no moving parts except for a liquid or liquids. This geophone or motion sensor has significantly better resistance to damage from mechanical shock and mechanical fatigue.
The sensor of the present invention is dependent on the flow of liquids in three dimensions. In a preferred embodiment, it comprises two tubular members, one inside the other, forming inner and outer chambers. The larger is closed at the bottom and top. The smaller tubular member is positioned coaxially and coincidently with the larger tubular member. The smaller tubular member has openings in or near the top and bottom so that liquid may flow between the two chambers. The chambers are partially filled with a single liquid or completely filled with two immiscible liquids of different density. The diameters of the chambers are chosen so that the mass of the liquid subject to capillary rise in one chamber differs from the mass of the liquid subject to capillary rise in the other chamber. When the sensor is placed in a gravitation field, capillary rise will cause the level of the liquid in the chambers to differ. If the mass subject to capillary rise in the inner chamber is larger, then for water and other liquids where the capillary rise is positive, the liquid in the inner chamber will rise above the level of the liquid in the outer chamber. For mercury and other liquids where the capillary rise is negative, the liquid in the outer chamber will rise above the level of the liquid in the inner chamber.
In the absence of acceleration, the liquid interface in the smaller inner chamber will, as a first order approximation, be a circle. As a second order approximation it may be visualized as a round shallow dish. Under vertical acceleration the liquid interface will move up in one chamber and down in the other depending on the sign or direction of the acceleration. With lateral acceleration the liquid interface will form, as a first order approximation, an ellipse. The projection of the major axis of the ellipse on to a horizontal plane will form a vector in the direction of the lateral acceleration. The magnitude of the lateral acceleration is defined by the angle between the major axis of the ellipse and the horizontal plane. As a second order approximation it may be visualized as an oval shaped shallow dish. The position and shape of the liquid interface in the smaller chamber may be measured to determine the three axis acceleration present.
The outer chamber will have a liquid interface like that of the smaller inner chamber except that it will be interrupted in the center by the inner tubular member. It may be visualized as a shallow dish with a hole in the middle. As with the inner chamber the position and shape of the liquid interface in the outer chamber may be measured to determine the acceleration present.
There are many ways to measure liquid position and thus determine the amplitude and frequency of motion with the motion sensor of the present invention. In a preferred embodiment this is accomplished by a photoemitter and a photodetector disposed at opposite ends of the inner chamber.